OC170 Mark 2: the self-assembling transistor

University of Groningen, Holland, scientists, together with colleagues from the University of Wuppertal and IBM Zurich, have developed a method to select semiconducting nanotubes from a solution and make them self-assemble on a circuit of gold electrodes – easing the difficulty of using carbon nanotubes to make very small electronic devices.

About 70 years ago, in Holland, the first production-type transistors got assembled by hand mostly by young women exerting great manual precision, yet happy with low wages and allowed to sing along in choir with tunes on the factory’s loudspeaker system. Now, University of Groningen, Holland, scientists, together with colleagues from the University of Wuppertal and IBM Zurich, have developed a method to select semiconducting nanotubes from a solution and make them self-assemble on a circuit of gold electrodes – easing the difficulty of using carbon nanotubes to make very small electronic devices.

The result of ten years of research, the method produces a self-assembled transistor with nearly 100 percent purity and very high electron mobility. Professor of Photophysics and Optoelectronics Maria Antonietta Loi designed polymers which wrap themselves around specific carbon nanotubes in a solution of mixed tubes. Thiol side chains on the polymer bind the tubes to the gold electrodes, creating the resultant transistor. These nanotubes can be depicted as a rolled sheet of graphene, the two-dimensional form of carbon.

Depending on the way the sheets are rolled up, they reportedly have properties ranging from semiconductor to semi-metallic to metallic. Only the semiconductor tubes can be used to fabricate transistors, but the production process always results in a mixture.

The researchers had the idea of using polymers with thiol side chains some time ago. As sulphur binds to metals, it will direct polymer-wrapped nanotubes towards gold electrodes. While Loi and his team were working on the problem, IBM even patented the concept. However, the polymers with thiols also attached to metallic nanotubes and included them in the transistors, which ruined them. Loi's solution was to reduce the thiol content of the polymers, with the assistance of polymer chemists from the University of Wuppertal. By using polymers with a low concentration of thiols, it is possible to selectively bring semiconducting nanotubes from a solution onto a circuit. The sulphur-gold bond is strong, so the nanotubes are firmly fixed: enough even to stay there after sonication of the transistor in organic solvents.